Solvent extraction of black liquor



United States Patent SOLVENT EXTRACTION F BLACK LIQUOR James E. Copenhaver, William A. Biggs, Jr., and William H. Baxley, Hartsville, S. C., assignors to Sonoco Products Company, a corporation of. South Carolina No Drawing. Application November 28; 1952, Serial No. 323,144

2 Claims. or. 260-527 This invention relates to the treatment of pulp and paper mill efiiuent to remove therefrom the constituents harmful to fish or other animal life in streams receiving the efiluent, and to recover in reusable form certain components of the waste eflluent, which is commonly referred.

semi-chemical or neutral sulfite process for paper pulp production. This type of process is becoming increasingly attractive and important because of the'high yields obtainable thereby and also the sheet-forming qualities of the pulp so produced. Paper mills are progressively adopting this semi-chemical process, and this fact has increased the problem of pollution in the streams into which the black liquor is discharged, as well as the economic necessity for recovering certain valuable constituents in the black liquor in a form suitable for commercial use. v

The black liquor resulting from the semi-chemical neutral sulfite process partciularly aggravates the stream-pollution problem because of the fact that the B. 0.. D. (biological oxygen demand) of the liquor solids from that type of process is approximately 400,000 parts per million. As a consequence, if'large quantities of black liquor produced in the semi-chemical process are discharged into streams containing fish and other marine life, it will cause deleterious results. Also, as above suggested, it is commercially important to recover from this" black liquor,

certain constitutents, especially the organicacids' and pentose and hexose sugars. A typical analysis of the neutral sulfite black liquor shows the following percentage of these and other solids presentthereiu: 5 v

The lignin content of the black liquor, indicated bythe above sodium lignin. sulfonate, has virtually no oxygen demand and therefore itself does not create a B. O. D;

problem. Nevertheless,,its decomposition yields mercap tans that produce very objectionableodors, and numerous efforts have been made and substantialsums of money expended in times past to alleviate this condition, with out complete success. I p

In accordance with the present invention, we have discoveredjth'at the B. O. D. problem can be satisfactorily I met and valuable recovery of the organic acids and sugars in the black liquor obtained, by acidifying the black liquor, and extracting the acidified black liquor with an ester- 2,744,927 Patented May 8, 1956 alcohol solvent mixture. Illustrative but non-limitingexamples of these mixtures are the following:

% ethyl acetate 10% ethanol 70% isopropyl acetate 30% isopropanol 75 normal propyl acetate 25% normalpropyl alcohol We have utilized the above mixtures, as well as other similar'ones, for solvent extraction of the acidified black liquor containing from about 10% to 60% solids, and usually about 25% to 40% solids, with very satisfactory results. As a result of this solvent extraction treatment of the black liquor, its B. O. D. value is reduced about 50%, and recoveries of about or more of the acetic and formic acid contents ofthe black liquor are obtained. Also, recoveries of about 10%. to 20% or higher of the pentose and hexose sugars are obtained. The higher the ratio of alcohol the higher the percentage removal of sugars.

It is important to note that the solvents usually accepted in the art for most eflicient recovery of acetic acid from dilute solutions are the aliphatic esters, such as ethyl, propyl, butyl, and amyl acetate- However, we found that in attempting to employ the known solvent extraction properties of these esters in the usual manner, that is, in the absence of alcohol, for treatment of black liquor, they were entirely ineffective and. unsatisfactory. When these solvents were used in essentially pure state, that is, 5% or less by'volume'of alcohol content, for extractingblack liquor, they-would completely emulsify upon agitation with the acid liquor, and accordingly extraction of the emulsified mixture could not be efiected by the usual gravity'procedures, .or, if so, only'after impractically long periods ofsettling. We foundthe same to be true ofthe higher molecular weight ketones, above methyl ethyl ketone, ethers, 'benzoh'toluene, carbon tetrachloride and alcoholsnotinfinitelysoluble in-water. All such materials producedanemulsion-which. was not practical to handle for extraction of: thefac'idszand sugar contents of the black liquor. These emulsions can be broken only by prolonged centrifuging which is impractical in a continuous extraction process and results in hard pre- 5 cipitation of suspended solids which rapidly foul the system;

We have been able to overcome all of these problems, in accordance with our above-mentioned discovery, by the combining of solvents having infinite water solubility, such" as acetone, methyl, ethyl, propyl, isopropyland tertiary butyl alcohols, with esters. having limited solubility, such as, for example, the above-mentioned ethyl and isopropyl acetates. This combination" of alcohols" and esters permits phase separation and prevents the formation of emulsions, in directproportion to the amount of alcohol used. This combination of esters and alcohols covers the lower aliphatic compounds and extends up to and includes the propyl isomers and tertiary butyl alcohol. I When such alcohols are combined with the lower aliphatic esters, the resulting solution proves highly effective as an extracting solvent for the solid'constitucuts of black liquor from the semi-chemical process. This combination solvent avoids the formation of an emulsion and gives good phase separation and high extraction efficiency, especially vvhen extracting blackliquor containing a relatively high solids concentration, for example, up to about 60% solids. We have found that the Also, we have noted that the higher the ratio of alcohol more concentrated the black liquor, the less likely it is to to ester, the greater the extraction efliciency per stage for emulsify, but at the same time when an emulsion is formed, acetic and formic acid and B. O. D. reduction. Test it is more difficult to break the emulsion than with lower data showing these relative values and indicating the charsolids concentrations. Similarly, We have found that 5 acteristic features of the process are given below. These the higher the molecular weight of the ester and the alcodata also illustrate the effect of alcohol, liquor solids, and hol, more alcohol is required in proportion to the ester molecular weight on the emulsion behavior, solids preto effect the separation, and the easier it is to emulsify. cipitation tendency, centrifuging, and extraction efficiency.

Table 1.-The extraction efiiciency and emulsification tendency of various solvents with acidified black liquor--One contact at 1/1 ratio by volume at 25 C.

[25 rule. solvent to 25 mls. liquor] Phase Separation Solubility Solid Pref g g Solvent Gravity Volumes centrifuged Volumes ggg? m ed Water In In Water 5 Percent Percent Ran. Extract Rafi. Extract mlx. mls. mls.

Emulsion- Emulsion. 26 24 31 3 0. 3 1. 3 6 25 25 1111 nil .03 0.1 38 12 1. 2 nil nil 30 20 2.3 nil 0 0.2 28 22 45. 2 0. 2 4 8 27 23 53. 6 0. 6 11 9 95% Ethanol 3 47 100. 0+ 3. w m 90% n-Butyl Acetate 1 27 23 38. 4 0.2 2 .5 Mixed Isomer Amyl Acetate 28 22 27. 8 0. 4 1. 2 2

Tertiary Butyl Alcohol 13 37 81. 3 0. 2 m n-Butyl AlcohoL. 22 28 60. 5 0. 2 20 8 WB-5299 2-Butanone 28 22 57. 1 0. 2 12 26 All the above was solids liquor (0.562 grams acid (as acetic) 25 m1s.). All those below were 40% solids liquor (2.25 grams Acid (as acetic) 25 mls) Wit-5271 Emulsi0n Emulsion. 3U 38.1 1.0 WB-5274 d0 .d0 26 24 5. 1 3.0 WB-5275 do do 26 24 9. 5 1. 1 WB-527 J -J 25 8. 5 1. 0 WB-6277 do do 25 25 58. 7 0. 9 WB-5278 22 R 23 27 73. 4 1. 3 WB-527' 25 25. 7 43 96.0 7. 4 WB-5580 Emulsion. Emulsion- 28 22 43. O 0. 6 WEI-528i "110"..." dO 26 24 40. 9 0.7 WB-5282 25. 25 20 30 75. 0 1. 0 WB-5283 Emulsion- Emulsion- 22 28 63. 2 1. 0 WB-5299 2-B11i'annnn 23. 28 22. 5 27 5 64. 3 0. 5

1 Remainder is corresponding alcohol.

Table 2.--Efiect of alcohols on the emulsification tendency and extraction efiiciency of esters with acidified black liquor at 25 C. (77 F.)

[25 rule. liquor/25 mls. solvent, 1 stage] ETHYL ACETATE-ETHANOL Liquor Solids 10% 25% (shaken) 40% (osterizedfi Experiment Number s WB-5255- WB-5254- WB-5256- WB-5257- Percent Alcohol in Ester 1 10 20 30 1 Extract Volume (m1s.):

After osterizing After shaking 2 min. and. standing 10 min 4 min and standing 10 min. 6 min. and standing 10 111111.. 8 min. and standing 10 min 8 min. extract after centrifuging 10 min. (mls.) 24 Extraction efficiency as acetic acid, percent:

to HHNI co oqtolo N NlONg cu OHM N NMMN sh unwind 03 M Q 000% m c: RONNNJ O OI-I-I-I are Solid precipitate on centrifuging (1111s.) nil Ultimate time to completely emulsify in osterizer (secsJ r, 12

O; CI 0 Main 2,742,927 '5 6 Table 2.Efiect. of alcohols on the. emulsification tendency and: extraction efliciency of. esters. with acidified]v black liquor at 25 C. (77 F.)Continued.

ISOP-ROPYL .AOETATE-ISOPROPANOL Liquor mirk 1 10% 25%- 40% Experiment Number WB-5259- WB'5260+- wn-szas- Percent Alcohol in Ester 5 10 20 30 5O 5 I 10; 20 30 5 10 20 30 50 Extract-Volume (mls.): 1

After shaking l 2 min. and standing 10 min 22 23. 5 23 23- O 1 28 4 min. and standing 10 min 18 19. 5 21 20 O 28 6 min and standing 10 min.- 0 0 2 18 0 0 28. 8 min. and standing 10 111111-- 0 I 0 14; 5-. 0 0 29 81211.5. extract-after centrifuging 10 min. 21 23 23. 5 26 4 29 s. Extraction efliciency as acetic acid, percent Notcentriiuged...- 0 0 49 0 0 33 6 0 81. 9 Centrifuged 32.7 41.3 48 8 53.6 40.6 44 1 55 4 50 81.9 Solid precipitate on centrifuging (mls.). nil nil 1i nil ml 0.1 O 1 0 1 1. 3 2.--0 Ultimate time to completely. emulsify in-osterizer.

(secs) I 9 9 9 9 15 32 w 42 125 m w n-PROPYL AOETATE-n-PROPANOL Liquor Solids.-....-. I Q 10% 3 Q 40% Experiment Number WB5261- WB-5262 WB-5263' Percent Alcoholin Ester 5 10 20 5 20 50 Extract Volume (mls.'):

After shaking- 2 min. and standing 10 min 0 22 23 0 0 0 4.min. and standing 10 min- 0 19 23 '0 0; 0 6 min. and-standing-ldminn 0 18 22 0' 0 0 8 min. and standing 10 min.- 0 15 18 0' 0 0 8 min. extract after centrifuging 10 min.

(mls. 23 24 24 5 17. 5 18 23 26 Extraction efficiency as acetic acid, percent: 4

Not centrifuged... 0 28. 3' 4 0 0 centrifuged 35 40.9 48, 36. 4 39. 3 Solid precipitate oncentrifugmg (mls.) nil r 1.1 1. 2 Ultimate time to completely emulsify in osterizer e v (secs.) 12 15: 36

1 Shaken in ml. tubes. v 3 Intensive Waring-type blender. 3 Viscosity too high for good mxing by shaking; can be compensated for by extraction at higher temperature. 4 01 clear extract. Where the extracted acids (acetic and formic) are to alcohol-water ternaries for each system. The presence be essentially dehydrated on solvent recovery, there is a of the acetic and formic acid will increase these solubilities limit to the amount of alcohol which may be used propor- 50 somewhat.

Table 3.-B0iling points of esters, alcohols, their binaries,

and ternaries with water, C.

[E==ester, A=a1cohol, W=water] Binary E-W Binary E-A Ternary B t 3 1 H20 .A,Per- E Per- A Per- W, Per- Perceiit cent cent cent cent Propyl formate 97. 2 81 71. 6 2. 3 80. 6 9. 8 70. 8 82 5 13 Butyl form-ate 117. 7 106. 9 83. s 16. 5 105. a 23. 7 s3. 6 0s. 7 10 21. 3 Isobutyl formate- 107. 9 9s. 5 s0. 4 7. s 97. s 20. 6 so. 2 6. 7 17. a Isoamyl iormate. 131. 6 123. 3 90. 2 21 123. 6 26 89. 8 48 19. 6 32. 4 Methyl acetate- 64. 6 54. 0 54 18. 5 Ethyl acetate... 78. 3 77. 1 70. 45 s. e 71. s 30. 0 70. 3 s3. 2 9 7. s Isopropyl acetate 77. 4 6. 2 76. 2 63. 7 26. 2 l0. 1

Propyl acetate... 97. 2 101. 0 s2. 4 14 94. 2 40 82. 2 59. 5 19. 5 21 Butyl acetate.. 117.7 125. 1 90.2 28 7 117.2 47 e9. 4 35.3 27.4 37.3 Isobutyl acetate-. 107. 9 110.3 87.4 16 s 87.4 16.6 86.8 40. 5 23.1 30.4 Amyl acetate 137.8 148 95. 2 41 94. s 10. 5 33. 3 5e. 2 Isoamyl acetate 131. o 139 .93. s 36. 2 93. 6 24 31.2 44. s

tionate to the amount of ester unless additional water en- The solvent extraction process may be carried out with trainers are used. This varies with the alcohol-ester sysa conventional tower utilizing counter-current flow printezn used. Table 3, below, gives the composition of esterciples for bringing the black liquor in contact with the water and alcohol-water binaries, as well as the esterester-alcohol extracting solvent. However, we have 0btractor manufactured by Podbielniak, Inc., Chicago, Illii nois. This extractor is covered in a number of patents issued to W. J. Podbielniak, including Patent No. 2,003,308, issued June 4, 1935. By use of this type of apparatus, accurate and instantaneous control can be had of the phase contacting and separation by application of highly variable degrees of centrifugal force balanced to just effect complete separation and suspended solids removal in the raflinate without fouling the extractor. Furthermore, on shut-downs of the extractor, the liquid holdup is only a few gallons against very large volumes in a counter-current gravity tower. Also, the use of this type apparatus reduces contact time between solvent and water phases to almost instantaneous contact and separation and thereby reduces solvent loss due to hydrolysis to a minimum. This is a very serious consideration when extracting with the lower esters in an acid system.

The following is an illustrative but non-limiting example of the process of this invention. Black liquor from the semi-chemical process is concentrated to solids content, this neutral concentrated black liquor is acidified with 66 Baum (95%) sulfuric acid, the acidified solution is passed through the heat exchanger to hydrolize the sugars, and the remaining solution is contacted countercurrently in the Podbielniak extractor with about 1 /2 volumes of the solvent combination of 90% ethyl acetate and 10% ethanol. The extract obtained is run through a solvent recovery column, and concentrated acetic and formic acids, containing a small percentage of solids dissolved from the liquor in the extraction operation, pass out of the bottom of the column. These concentrated acids containing dissolved solids are run through a rectifying column. The acetic and formic acids are taken as overhead and furfural containing dissolved solids is passed out of the bottom of the column and is subsequently distilled to remove water and is then drawn 0E and passed to storage.

The same process described above has been carried out under the same conditions by using in the different cases the following ester-alcohol solvent combinations:

70% isopropyl acetate isopropanol 75% n-propyl acetate 25 n-propanol 75% n-propyl acetate 25% acetone The above solvent ratios and other conditions in the process may be varied to obtain best results with various concentrations of the black liquor, temperature of extraction, centrifugal force applied, including gravitational force, intensity of phase mixing, and other operating conditions. Likewise, the volume ratios of black liquor to solvent are not limited to-examples cited and will depend upon other variables including slope of the equilibrium line, stage efficiency, and the degree of extraction desired.

We claim:

1. A process of treating black liquor to efiect the removal therefrom of acetic acid, comprising concentrating said black liquor to a solids content of approximately 25% to acidfying the concentrated black liquor with sulfuric acid, and extracting the concentrated and acidified black liquor with a solution composed of isopropyl acetate and 30% isopropanol and separating the acetic acid from the extracting solvent.

2. A process of treating black liquor to effect the removal therefrom of acetic acid, comprising concentrating said black liquor to a solids content of approximately 25% to 40%, acidifying the concentrated black liquor with sulphuric acid, and extracting the concentrated and acidified black liquor with a solution composed of approximately 70% isopropyl acetate and approximately 30% isopropanol, and separating the acetic acid from the extracting solvent.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,684 Gordon et al. Sept. 13, 1938 OTHER REFERENCES Doering: Chem. Abstracts, vol. 38, column 4128 1944).

McLaughlin: Chem. Abstracts, vol. 43, column 7682 (1949).

Braun et al.: Chem. Abstracts, vol. 44, column 4677 (1950). 

1. A PROCESS OF TREATING BLACK LIQUOR TO EFFECT THE REMOVAL THEREFROM OF ACETIC ACID, COMPRISING CONCENTRATING SAID BLACK LIQUOR TO A SOLIDS CONTENT OF APPROXIMATELY 25% TO 60%, ACIDIFYING THE CONCENTRATED BLACK LIQUOR WITH SULFURIC ACID, AND EXTRACTING THE CONCENTRATED AND ACIDIFIED BLACK LIQUOR WITH A SOLUTION COMPOSED OF 70% ISOPROPYL ACETATE AND 30% ISOPROPANOL AND SEPARATING THE ACETIC ACID FROM THE EXTRACTING SOLVENT. 